1. Field of the Invention
The invention relates generally to a semiconductor circuit, and more particularly to a circuit for producing poly-phase quadrature signals necessary in high frequency transmitter and receiver of a communication system, in which the quadrature signals are produced using the phase difference between a load having a low-pass filter characteristic and a load having a high-pass filter characteristic and the quadrature signal is then used in the differential structure to produce amplified signal having 4 quadrature phases.
2. Description of the Prior Art
In a digital communication system, generally, if the frequency band of a signal to be transmitted does not match with the property of a medium, it is required that the signal be moved to an adequate frequency band and then transmitted. Among modulation methods, as identifying the phase difference of 180xc2x0 is much easier than identifying variations in the frequency, phase shift-keying (PSK) technique has been widely employed. In a modern high-speed communication system for processing a lot of data, quadrature phase shift-keying(QPSK) technique which four values can be identified by a single symbol with phase shift by 90xc2x0 has been widely used. In this QPSK technique, the phase shift by 90xc2x0 of a series of binary data pair can be generated by a pair of a mixer using a carrier wave of a cosine component and a mixer using a carrier wave of a sine component. At this time a poly-phase quadrature signal generator produces the quadrature signals of sine wave and cosine wave.
FIG. 1a shows a conventional poly-phase quadrature signal generator of the most simple structure. The poly-phase quadrature signal generator includes two resistors 104 and 107, and two capacitors 105 and 106. If differential input signals vin (0xc2x0), vin (180xc2x0) are inputted through input terminals 100 and 101, differential output signals vout (90xc2x0), vout (270xc2x0) are produced through the output terminals 102 and 103. At this time, assuming that R1=R2=R and C1=C2=C, the transfer function between the input and the output is expressed as follows: Vout*/Vin*=(1xe2x88x92sRC)/(1+sRC). As a result, phase shifts by xe2x88x922 tanxe2x88x921xcfx89RC. Vin* and Vout* represent the differential input signals vin (0xc2x0), vin (180xc2x0) and vout (90xc2x0), vout (270xc2x0), respectively. s indicates jxcfx89 and xcfx89 indicates an angular frequency. Therefore, when xcfx89=1/RC, the output is the differential quadrature signal (poly-phase quadrature signal) having same amplitude and phase difference of 90xc2x0 compared with the input. In a typical integrated circuit manufacturing process, however, there exist errors in the amplitude and phase as the range in an error of the resistor and the capacitor is high. In order to correct the errors between the quadrature signals, therefore, it is required that a variable resistor or a variable capacitor be used.
Hereinafter, signals having relative phases of 0xc2x0 and 180xc2x0 are called I-signal (in-phase signal) and signals having relative phases of 90xc2x0 and 270xc2x0 are called a Q-signal (quadrature-phase signal), according to common high frequency communication terminology.
FIG. 1b illustrates a conventional poly-phase quadrature filter. Resistors 118xcx9c121 and capacitors 126xcx9c129 form a first order filter network. Also, resistors 122xcx9c125 and capacitors 130xcx9c133 form a second order filter network. However, a high order filter network can be designed depending on its purposes. If the values of the resistors forming respective filter networks are same and are called R, and the values of the capacitors are same and are called C, the transfer function of the output signal to the differential input signal vin (0xc2x0), vin (180xc2x0) when the differential input signal vout (90xc2x0), vout (270xc2x0) is grounded can be expressed as Equation 1.
VIxe2x80x94out*/Vin*=2sRC/{(sRC)2+4sRC+1},
VQxe2x80x94out*/Vin*={1xe2x88x92(sRC)2}/{(sRC)2+4sRC+1}xe2x80x83xe2x80x83[Equation 1]
where, VIxe2x80x94out* is the differential output signal, vout (0xc2x0), vout (180xc2x0), in other words, the I-signal. Also, VQxe2x80x94out* is the differential output signals, vout(90xc2x0) and vout(270xc2x0), in other words, the Q-signal. As the poly-phase quadrature filter is basically a passive circuit network, attenuation of the signal becomes great as the order of the circuit network is increased. The errors of the amplitude and the phase of the differential quadrature signal are also significantly increased by errors in the manufacturing process of the resistors and the capacitors. As shown in Equation 1, the I-signal represents a low-pass characteristic and the Q-signal represents a high-pass characteristic. Thus, as the frequency characteristics of the two differential quadrature signals are different, there exists difference in the modulation of the I-signal and the Q-signal by the low-frequency coupling or the high-frequency coupling due to the leakage, which causes errors in the amplitude and the phase. In order to generate exact differential quadrature signal in a desired frequency range, therefore, there is a need for a quadrature signal generator having a bandpass characteristic through which corresponding frequency components are passed but unnecessary signal by the leakage are rejected.
FIG. 1c shows a modified version of the poly-phase quadrature signal generator in FIG. 1a. Transistors 141xcx9c144 which are bipolar junction transistors(BJT) operate as an input buffer. A P type MOSFET Qc1 and Qc2 which are driven in a linear region, and resistors R1 and R2, are connected in parallel to form variable resistors 134 and 135. Thus, the gate voltage of the P type MOSFET is varied to control the effective resistor value, so that errors in the amplitude and the phase between two differential quadrature signals(I-signal and Q-signal) can be controlled. At this time, necessary control voltage is supplied from the outside through a node 136. As this type of the structure basically includes the resistors and the capacitors, the pole frequency is increased as the frequency is increased. Therefore, desired values of the resistors and capacitors become small. Consequently, there are problems that the structure could not be reliably implemented and implementation at the high frequency band is limited.
The present invention is contrived to solve the above problems and an object of the present invention is to provide a quadrature signal generator by which amplified quadrature signals are produced using a load having a low-pass filter characteristic using resistors and capacitors and a load having a high-pass filter characteristic using the resistors and inductors in a signal amplifier structure and variation in the phase representing respective loads, and quadrature signals are produced by implementing it as the differential structure, thus controlling amplitude errors and phase errors using a variable resistor and a variable condenser and removing signal errors(amplitude errors, phase errors) by coupling the bandpass resonators.
Another object of the present invention is to provide a quadrature signal generator that can be used in microwave and millimeter-wave regions as well as the radio frequency(RF) region, using an inductor other than an existing resistors-the capacitor structure.
In order to accomplish the above object, a quadrature signal generator according to the present invention, comprising a means for generating two-phase quadrature signals using a low-pass filter having resistors and a capacitors and a high-pass filter having resistors and a inductors, and a differential amplifier for generating amplified poly-phase quadrature signals, wherein the low-pass filter and the high-pass filter are connected to respective loads.
The values of the resistors and the capacitor in the low-pass filter are varied depending on the amplitude and phase of signal to be obtained and wherein the values of the resistors in the high-pass filter are varied depending on the amplitude and phase of a signal to be obtained.
The active quadrature signal generator further includes a resonator having a bandpass characteristic connected between the input terminals of the differential amplifier and having capacitors and inductors. The active quadrature signal generator further includes a poly-phase quadrature filter connected between of poly-phase quadrature signal output terminals and having passive devices. The active quadrature signal generator further includes a phase control means for controlling the values of the resistors in the high-pass filter in order to finely control the phase error of the poly-phase quadrature signal.
The phase control means includes a phase sensing means of a single parallel structure for sensing the difference in the phases between a relative phase 0xc2x0 and a relative phase 90xc2x0 or a relative phase 180xc2x0 and a relative phase 270xc2x0 among the poly-phase quadrature signals; a low-pass filter for removing a high frequency portion from the signals outputted from the phase sensing means; an averaging circuit for adding by weight a single output signal passed through the phase sensing means for sensing the difference in the phase between the relative phase 0xc2x0 and the relative phase 90xc2x0 and the low-pass filter, and a single output signal passed through the phase sensing means for sensing the difference in the phase between the relative phase 180xc2x0 and the relative phase 270xc2x0 and the high-pass filter; and a comparator circuit for comparing the output signal from the averaging circuit and a reference voltage to produce a voltage for controlling the resistors and the capacitors.
The phase control means includes a phase sensing means of a dual balanced structure capable of sensing the difference in the phases between the relative phase 0xc2x0 and the relative phase 90xc2x0 or the relative phase 180xc2x0 and the relative phase 270xc2x0 among the poly-phase quadrature signals; a low-pass filter for removing a high frequency portion of the signal outputted from the phase sensing means; a means for converting the differential output signal from the phase sensing means and the low-pass filter into a single output signal; and a comparator circuit for comparing the output signal from the means and a reference voltage to produce a voltage for controlling the resistors and the capacitor.
Further, an active quadrature signal generator according to the present invention, is characterized in that it comprises a first means for in parallel distributing the signal having a relative signal 0xc2x0 passed through a load having a low-pass filter characteristic into a load having a low-pass filter characteristic and a load having a high-pass filter characteristic to produce signals having a relative signal 0xc2x0 and a relative signal 270xc2x0; a second means for in parallel distributing a signal having a relative signal 180xc2x0 passed through the load having the low-pass filter characteristic again into the load having the low-pass filter characteristic and the load having the high-pass filter characteristic to produce signals having the relative signal 90xc2x0 and the relative signal 180xc2x0; a third means for differentially connecting the first and second means; a fourth means for in parallel distributing the signal having the relative signal 90xc2x0 passed through the load having the high-pass filter characteristic again into the load having the low-pass filter characteristic and the load having the high-pass filter characteristic to produce signals having the relative signal 270xc2x0 and the relative signal 90xc2x0; a fifth means for in parallel distributing the signal having the relative signal 270xc2x0 passed through the load having the high-pass filter characteristic again into the load having the low-pass filter characteristic and the load having the high-pass filter characteristic to produce signals having the relative signal 0xc2x0 and the relative signal 180xc2x0; and a sixth means for differentially connecting the fourth and fifth means.
The active quadrature signal generator further includes a means for adding by weight the signal passed through the load having the high-pass characteristic having the same relative phase to the signal passed through the load having the low-pass characteristic.